A famous study followed hundreds of men who graduated from Harvard from 1939 to 1944 throughout their lives, into their 90s. The researchers wanted to know who flourished, who didn’t, and the decisions they had made that contributed to that well-being. The lead scholar on the study for many years was the Harvard psychiatrist George Vaillant, who summarized the results in his book Triumphs of Experience. Here is his summary, in its entirety: “Happiness is love. Full stop.”

The current director of the study, the psychiatrist Robert Waldinger, filled in the details. He said in a recent interview that the subjects who reported having the happiest lives were those with strong family ties, close friendships, and rich romantic lives. The subjects who were most depressed and lonely late in life—not to mention more likely to be suffering from dementia, alcoholism, or other health problems—were the ones who had neglected their close relationships.